The invention relates generally to the field of polymeric optical articles. More particularly, the invention concerns polymeric optical materials and articles, such as plastic lenses, that maintain stable performance characteristics over a broad temperature range.
Plastic lenses and glass lenses often perform the same function in optical systems, such as in cameras, microscopes, telescopes and opthalmic wear. The two main attributes that separate plastic lenses from glass lenses are cost and optical stability. Plastic lenses typically cost {fraction (1/100)}th the price of a similar glass lens. On the other hand, the stability of the refractive index of a glass lens with respect to temperature and humidity is typically 100 times better than that of a plastic lens.
The difference in cost is due largely to the difference in manufacturing processes that are required for the two materials and the relative temperatures that the materials must be formed at. Plastic lenses are typically produced at 230xc2x0 C. using injection molding at cycle times that are 10 times faster than glass lenses that are largely produced by grinding and polishing or compression molding at 625xc2x0 C. Grinding and polishing are labor intensive while the high temperatures that glass must be formed at requires expensive mold materials and extensive maintenance costs.
In contrast, the difference in optical stability between plastic and glass is due to differences in their basic material properties. This difference in optical stability results in substantially more variation in focus and image quality in articles such as cameras when plastic lenses are used in place of glass. What is desired, and a remaining challenge in the art, is a material with the optical stability of glass that processes like a plastic. While optical plastic materials such as cyclic olefins greatly improve the refractive index stability with respect to humidity, improving the refractive index stability with respect to temperature has remained an opportunity. A study on the competing fundamental material characteristics that determine the sign and the magnitude of the dn/dT of glasses is available, for instance, by Lucien Prod""homme, xe2x80x9cA new approach to the thermal change in the refractive index of glasses,xe2x80x9d Physics and Chemistry of Glasses, Vol. 1, No. 4, Aug. The two competing effects that determine the dn/dT in glasses are the density change which produces a negative dn/dT and the electronic polarizability which produces a positive dn/dT. The net dn/dT in a glass material depends on which effect dominates. In optical plastics however, there is not an electronic polarizability so that all unfilled materials have negative dn/dT values. Nonetheless, the article by Prod""homme does identify the possibility of using glass-like fillers with positive dn/dT values to substantially alter the dn/dT of a filled plastic composite material.
Nanoparticulate fillers have been used to modify the index of refraction of optical plastics. By using a filler small enough that it is well below the wavelength of visible light (400-700 nm), the filler will not scatter the light and the filled plastic can retain its transparency. WIPO Patent WO97/10527 describes the use of nanoparticles to increase the refractive index of plastics for opthalmic applications. In addition, technical references that describe the addition of nanoparticles to increase the refractive index of plastics include: C. Becker, P Mueller, H. Schmidt; xe2x80x9cOptical and Thermomechanical Investigations on Thermoplastic Nanocomposites with Surface-Modified Silica Nanoparticles,xe2x80x9d SPIE Proceedings Vol. 3469, pp. 88-98, July 1998; and, B. Braune, P. Mueller, H. Schmidt; xe2x80x9cTantalum Oxide Nanomers for Optical Applications,xe2x80x9d SPIE Proceedings Vol 3469, pp. 124-132, July 1998. While these references disclose the use of nanoparticles to modify refractive index of optical plastics they do not discuss the issue of refractive index stability with respect to temperature which requires a different set of characteristics in the nanoparticle.
U.S. Pat. No. 6,020,419 issued to M. Bock, et al., discloses the use of nanoparticulate fillers in a resin based coating for improved scratch resistance. U.S. Pat. No. 5,726,247 issued to M. Michalczyk, et al., also describes a protective coating that incorporates inorganic nanoparticles into a fluoropolymer. While scratch resistance is important in plastic optics, the nanoparticles that would be suitable for scratch resistance would be very different from those with the specific properties needed to improve refractive index stability with respect to temperature.
U.S. Pat. No. 3,915,924 issued to J. H. Wright describes a nanoparticulate filled clear material for filling voids. U.S. Pat. No. 5,910,522 issued to H. Schmidt, et al., describes an adhesive for optical elements that includes nanoscale inorganic particles to reduce thermal expansion and improved structural properties at elevated temperatures. While the inventions described in these patents represents some progress in the art, none of them address specific optical properties of the modified plastic material particularly as these properties relate to temperature sensitivity.
WIPO Patent WO99/61383A1 a discloses a method for producing multilayered optical systems that uses at least one layer that contains nanoparticulate fillers to form a layer with a different refractive index than the substrate to create an interference filter or an antireflection layer. Obviously, this patent is addressing another form of modification of the index of refraction and such is not concerned with the stability of the index of refraction with respect to temperature.
Skilled artisans will appreciate that a wide variety of materials are available in nanometer particle sizes that are well below the wavelength of visible light. Representative materials may be acquired from companies such as Nanophase Technologies Corporation and Nanomaterials Research Corporation. By selecting nanoparticle materials based on properties other than index of refraction, our experience indicates that it is now possible to modify other optical properties of plastics.
While there have been several attempts to modify properties of plastics using nanoparticles, none of these attempts have proven successful in producing optical plastic articles with temperature stable optical properties while retaining important processing characteristics.
Therefore, a need persists in the art for optical plastic articles, such as lenses, and a method of making same that have temperature stable optical properties.
It is, therefore, an object of the invention to provide an optical nanocomposite material that has reduced temperature sensitivity.
Another object of the invention is to provide an optical article, such as a plastic lens, that maintains stability over a broad range of temperatures.
Yet another object of the invention is to provide a method of manufacturing an optical article having reduced temperature sensitivity.
It is a feature of the optical article of the invention that a select nanoparticulate dispersed into a plastic host material having a temperature sensitive optical vector that is directionally opposed to the temperature sensitive optical vector of the nanoparticulate filler.
To accomplish these and other objects, features and advantages of the invention, there is provided, in one aspect of the invention, a polystyrene nanocomposite optical plastic article comprising: a polystyrene host material having a temperature sensitive optical vector x1 and nanoparticles dispersed in said polystyrene host material having a temperature sensitive optical vector x2, wherein said temperature sensitive optical vector x1 is directionally opposed to said temperature sensitive optical vector x2.
In another aspect of the invention, there is provided a method of manufacturing a polystyrene nanocomposite optical plastic article, comprising the steps of:
(a) providing a polystyrene host material having a temperature sensitive optical vector x1 and nanoparticles having a temperature sensitive optical vector x2, wherein said temperature sensitive optical vector x1 is directionally opposed to said temperature sensitive optical vector x2,
(b) dispersing said nanoparticles into said polystyrene host material forming a polystyrene nanocomposite material; and,
(c) forming said polystyrene nanocomposite material into said polystyrene nanocomposite optical plastic article.
Hence, the present invention has numerous advantageous effects over existing developments, including: (1) the resulting nanocomposite has a significantly lower dn/dT (change in refractive index with temperature); (2) lenses made with the nanocomposite material have more stable focal length over a given temperature range; (3) low levels of dn/dT are achievable in the nanocomposite material with reduced loading of the nanoparticulate; (4) the viscosity of the nanocomposite material is not significantly higher than the base plastic so that conventional plastic processing techniques can be used; and, (5) the nanocomposite material has improved barrier properties so that the change of refractive index with respect to humidity will be reduced compared to the base plastic.